The invention relates to a method of growing an amorphous silicon film and a method of fabricating a semiconductor device involving an amorphous silicon film, and more particularly to an improvement in a quality and a growth rate of an amorphous silicon film.
Amorphous silicon is useful for a thin film transistor or other switching devices as being grown to extend in a large area at a low temperature as well as possessing a high resistivity which is required to reduce an off-current. Such a thin film transistor is usable in active matrix liquid crystal display devices or in image processing devices. Recently, for semiconductor devices such as thin film transistors involving the amorphous silicon film, requirements in reduction of manufacturing cost thereof and improvement in a quality thereof are considerable. Achievement of the reduction of the manufacturing cost of the semiconductor device involving the amorphous silicon requires an improvement in a growth rate of the amorphous silicon, while permission of the device to have a high quality and high reliability requires an improvement in a quality of the amorphous silicon. To comply with the above both requirements, it is required to develop a method of growing the amorphous silicon with an excellent quality even if the amorphous silicon is grown at a high growth rate.
It has generally been known in the art that the amorphous silicon film may be grown by use of a plasma chemical vapor deposition method in which a continuous high frequency discharge or successive radio frequency discharge is carried out for decomposition of silane system gases. Waveform of the discharge is as illustrated in FIG. 1. This continuous high frequency discharge plasma chemical vapor deposition method may cause no deposition of part of a hydrogenated amorphous silicon on a substrate and a deposition of cluster hydrogenated amorphous silicon thereon under a condition of a high deposition rate of the hydrogenated amorphous silicon. Such non-deposited or cluster hydrogenated amorphous silicon may form powder particles which cause a remarkable deterioration in the quality of the deposited hydrogenated amorphous silicon film.
The above problem with the conventional chemical vapor deposition method by use of the continuous high frequency discharge is as follows. Increase of the power density of the high frequency discharge not only results in an improvement in a high deposition rate of the hydrogenated amorphous silicon on the substrate for reduction of the manufacturing cost but also leads to a generation of powder particles formed of the non-deposited or cluster hydrogenated amorphous silicon.
To settle the above problems, it was proposed to use a discontinuous high frequency discharge in the chemical vapor deposition for a high speed deposition of the hydrogenated amorphous silicon on the substrate to obtain a powder particle free amorphous silicon film. One of the conventional discontinuous high frequency discharge deposition methods for the hydrogenated amorphous silicon is disclosed in Technology Reports of Kyushu University, Vol. 62, No. 6, December 1989 "Reaction Control in Processing Plasma by Square-Wave-Amplitude-Modulating RP Voltage". This publication reported that it has been shown experimentally that voltage-modulation in radio frequency discharging SiH.sub.4 gas bring about drastic suppression of power generation all over the discharge space and also about enhancement of an electron density and a deposition rate of hydrogenated amorphous silicon in a specific range of discharge frequency. In order to clarify such phenomena, reaction processes are studied in a He+SiH.sub.4 plasma produced by a square-wave amplitude modulation radio frequency discharge. This results in that the enhancement of electron density is due to a dissociative recombination process of molecular ions such as SiH.sub.n (n=1 to 3), the suppression of powder generation is due to slow formation and fast extinction processes of the powder, and ratios between SiH.sub.3 and SiH.sub.n (n=0 to 2) densities become fairly large in the radio frequency power-off period.
Whereas the publication merely recites the basic mechanisms of the high rate deposition of the hydrogenated amorphous silicon by use of the discontinuous high frequency discharge deposition method, the publication neither recites nor suggests concretely experimental results as to the deposition rate and the quality of the resultant amorphous silicon film.
Another one of the conventional discontinuous high frequency discharge deposition methods for the hydrogenated amorphous silicon was reported by J. Togawa et al. in Japan Applied Physics Conference 1993 as entitled "High Rate Deposition of Hydrogenated Amorphous Silicon Film by PE-CVD System" which discloses that a high rate deposition of the hydrogenated amorphous silicon is carried out by use of the square wave amplitude modulated radio frequency discharge to suppress a generation of powder. This recites a relationship of the deposition rate versus duty ratio of the high frequency discharge. The powder generations are reduced by the high frequency modulated discharge.
Although the publication merely recites the basic mechanisms of the high rate deposition of the hydrogenated amorphous silicon by use of the discontinuous high frequency discharge deposition method, the publication neither recites nor suggests concretely experimental results as to the deposition rate and the quality of the resultant amorphous silicon film.
Further, the Japanese laid-open patent application No. 60-50171 describes a chemical vapor deposition system in which discontinuous high frequency plasma discharge is carried out to decompose source gasses such as silane for the high rate deposition of the hydrogenated amorphous silicon.
While the publication merely recites the basic mechanisms of the high rate deposition of the hydrogenated amorphous silicon by use of the discontinuous high frequency discharge deposition method, the publication neither recites nor suggests concretely experimental results as to the deposition rate and the quality of the resultant amorphous silicon film.
Moreover, it is disclosed in the Japanese Laid-open Patent Application No. 58-157600 to utilize the discontinuous high frequency plasma discharge for decomposition of the source gases such as silane to achieve a high rate deposition of the hydrogenated amorphous silicon on the substrate. The discharge operation is controllable by gate pulse operation. When the gate pulse is in ON state, the high frequency plasma discharge is carried out to decompose the source gas for a high rate deposition of the hydrogenated amorphous silicon on the substrate. When the gate pulse is in the OFF state, the plasma discharge is discontinued. Time period and duty ratio of the discontinuous high frequency plasma discharge are constant as illustrated in FIG. 2.
While the publication merely recites the suppression of the powder generation on the basis of the basic mechanisms of the high rate deposition of the hydrogenated amorphous silicon by use of the discontinuous high frequency discharge deposition method, the publication neither recites nor suggests concretely experimental results as to the deposition rate and the quality of the resultant amorphous silicon film.
Furthermore, it is disclosed in Applied Physics Letters, Vol. 57, No. 16, October, 1990, pp. 1616-1618 to utilize a discontinuous radio frequency modulated plasma discharge for high speed chemical vapor deposition of the hydrogenated amorphous silicon on the substrate. A high deposition rate of 360 angstroms per minute for hydrogenated amorphous silicon deposition on the substrate under the conditions of a low concentration of 5% SiH.sub.4 and a high radio frequency peak power of 200 W results in an absence of any appreciable amount of powder particles in a reaction chamber.
In view of the mass production of the semiconductor devices involving the amorphous silicon film, further improvement in the deposition rate of the hydrogenated amorphous silicon free from any powder particle has been required for improvement in the throughput of the plasma chemical vapor deposition system to curtail the manufacturing cost of the semiconductor device. In the above viewpoint, for the plasma chemical vapor deposition, higher deposition a rate of at least 400 angstroms per minute free from any generation of powder particle is required.
Still further, the amorphous silicon is useful in the thin film transistor as described above. It is very important to improve a deposition rate for deposition of a high quality amorphous silicon film free from any powder particle to curtail the manufacturing cost of the thin film transistor involving the amorphous silicon film. One of the conventional thin film transistors involving the amorphous silicon film is disclosed in the Japanese laid-open patent application No. 64-71173 in which the thin film transistor has a high electron mobility property. A structure of the thin film transistor is illustrated in FIG. 3. The thin film transistor has an insulative substrate 11 on which a gate electrode 12 is formed. A gate insulating film 13 is formed on the gate electrode and the insulating substrate 11. A high electron mobility amorphous silicon film 61a having a thickness of 20 nanometers is deposited at a low deposition rate on the gate insulating film 13 by a high frequency discharge at a low power. A low electron mobility amorphous silicon film 61b having a thickness of 300 nanometers is deposited at a high deposition rate on the high electron mobility amorphous silicon film 61a by a high frequency discharge at a high power. The low electron mobility amorphous silicon film 61b has a recessed portion surrounded projecting top portions. A doped layer 15 is formed on the top portions of the low electron mobility amorphous silicon film 61b. Source and drain electrode 16 and 17 are provided on the doped layer 15. The deposition of the high electron mobility amorphous silicon film 61a is carried out at a deposition rate of 5 nanometers per minute for four minutes, while the deposition of the low electron mobility amorphous silicon film 61b is carried out at a deposition rate of 30 nanometers per minute for ten minutes. The deposition rate of 30 nanometers per minute for deposition of the low electron mobility amorphous silicon film is almost the maximum deposition rate to suppress any generation of powder particles for high quality of the amorphous silicon film.
To facilitate the reduction of the manufacturing cost, it is important to shorten the time necessary for deposition of the amorphous silicon film involved in the thin film transistors. This requires increasing the deposition rate of the amorphous silicon films involved in the thin film transistors, while it is necessary for securing a high quality of the thin film transistor to suppress any generation of powder particles in deposition of the amorphous silicon films. As described above, the generation of the powder particles in the amorphous silicon deteriorates the quality of the amorphous silicon film. This results in a low yield in manufacturing the thin film transistor.
In the viewpoints as described above, it would therefore be required to develop a novel method for further increase of a deposition rate in a high frequency plasma discharge chemical vapor deposition of a powder particle free amorphous silicon film for considerable reduction of a manufacturing cost of semiconductor devices involving the amorphous silicon film as well as a high quality thereof.